This application is a 371 of PCT/EP99/02597, which was filed on Apr. 17, 1999.
The invention relates to a method for carrying out photochemical and thermochemical solar syntheses by means of flat-bed solar light collectors without devices for focusing or concentrating the sunlight and does not need sun tracking.
Several solar technical devices have been disclosed in DE 4134614 C2 for the conduction of solar photochemical reactions wherein the solar radiation does not reach the reaction medium directly, but only upon passing an auxiliary device for the concentration of the light. Since the emitted solar radiation is, dependent on geografical situation and weather, differently dispersed as diffuse radiation and is hence only partially available for concentration purpose. Therefore, light concentrating solar devices leave a considerable portion of the solar radiation for photochemical reactions unused. It is emphasized in DE 43 44163 Al that less by-products are obtained at higher conversions in the photooxidation of terpene olefins if working with high concentration of radiation.
Otherwise, the application of concentrating or focussing devices is at our geographical latitudes, lacking direct solar radiation, disadvantageous for (photo)chemical transformations.
Surprisingly, it has now been found that this aggravating disadvantage using solar radiation energy for photo- and thermochemical purpose can be avoided by employing a technically simpler and hence cheaper flat-bed solar light collector since it is suited to use diffuse solar radiation as well as artificial light without significant changes of product ratios in the reactions carried out under these conditions. Furthermore, the combination of solar and arificial (for night runs) light allows continuous production. Further, it was found that the claim by Scharf (Angew. Chem. Int. Ed. Engl. 33, 2009 (1994)) that concentration of sunlight gives rise to higher volume-to-time yields is erroneous. In contrast, at our geographical latitudes the volume-to-time yields are even higher when using a flat-bed solar light collector than by employing a parabolic trough concentrator. The essential use of flat-bed solar light collectors has so far been for detoxification, particularly for air and water, i.e. for the decomposition of low-molecular-weight compounds. We found that flat-bed solar light collectors are specially suited for the synthesis (building-up) of stereochemically and structurally complex compounds of higher molecularity.
For the purpose of satisfactory product yields, sun-tracking of a flat-bed solar light collector is not necessary establishing a low-tech/low-price technology.
Especially advantageous is a mirrored surfacexe2x80x94advantageously as a surface-structured and radiation-reflecting metal or metal-coated plastic foilxe2x80x94at the inner site of the flat-bed solar light collector, e.g. at the bottom of the reactor (FIG. 1). In this way the solar radiation, which by penetrating the reaction media in direction of the bottom of the reactor is not absorbed and does therefore not lead to photochemical reactions, is further available by reflexion. In order to adjust the wavelength of the incident solar radiation to the individual demand of the reaction, the reactor can on the top be tightly (against solvents) covered with a transparent/UV-transparent glass or preferentially with a plastic foil. While carrying out photochemical reactions, proceeding sluggishly with UV light or light of other wavelengths, additional covering of the previously mentioned foil by an e.g. UV non-transparent layer can filter off the UV part or other wavelengths of the incident radiation.